Calcium Nutrition and Resistance of Alfalfa to Ditylenchus dipsaci

Stem nematode-susceptible 'Atlantic' and resistant 'Lahontan' alfalfa seedlings, grown in sand and watered with complete nutrient solutions containing 0.75, 1.5, 3.0, 6.0, or 12.0 mM Ca/liter, were inoculated with Ditylenchus dipsaci (the stem nematode) 5-6 days after emergence. Approximately equal numbers of nematodes entered the tissues of each variety/Ca concentration within 2 days. Penetration was reduced at 12 mM Ca/liter. Reproduction during 21 days following inoculation yielded 3-fold, or greater, nematode increases in 'Atlantic' buds at all Ca concentrations, in 'Atlantic' cotyledons at the four lower concentrations, in 'Lahontan' buds at the lowest concentration and in 'Lahontan' cotyledons at the two lowest concentrations. Reproduction was lower at the higher Ca concentrations.Increased nutrient Ca concentrations resulted in increased Ca content, decreased Na and K content, and unchanged Mg content of buds and cotyledons. Accordingly, increased nutrient Ca resulted in increased divalent/monovalent cation ratios (Ca + Mg/Na + K ). Resistance to reproduction was correlated more closely with the divalent/monovalent cation ratio than with Ca content of tissue, At the four higher nutrient Ca concentrations, 'Lahontan' buds had higher ratios than 'Atlantic,' and infected buds had higher ratios than noninfected buds. Although cation balance modifies disease expression, the basic resistance mechanism remains unknown.